Radio communication system and apparatus

ABSTRACT

A radio communication system for transmitting a radio signal with a transmission format in which a channel response calculation preamble signal serving as a reference upon reception is inserted is disclosed. The output frequency of channel response calculation preamble signals is varied in accordance with a temporal change in radio propagation environment. A channel response variation measurement unit measures a variation of the radio propagation environment. A channel response calculation preamble signal output frequency setting unit sets the output frequency of preamble signals in accordance with the temporal change in radio propagation environment. The preamble signals are inserted in the transmission format to be transmitted at that frequency.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present continuation application claims the benefit of priorityunder 35 U.S.C. §120 to application Ser. No. 10/102,835, filed on Mar.22, 2002, and under 35 U.S.C. §119 from Japanese Application No.2001-087040, filed on Mar. 26, 2001, the entire contents of both arehereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a radio communication apparatus, systemand method for transmitting a radio signal in accordance with atransmission format in which a channel response calculation preamblesignal serving as a reference is inserted.

2. Description of the Related Art

In recent years, a radio data communication system which can makehigh-speed data communications indoors or outdoors is required. In aradio communication system that implements high-speed datacommunications, distortions due to multi-pass interference, i.e.,reception of a transmitted signal via various routes due to reflectionby buildings, and the like, must be compensated for.

When a transmitter transmits a known reference signal (channel responsecalculation preamble signal), a receiver calculates the channel responseof the channel response calculation preamble signal. The channelresponse indicates the degree of distortion of phase, amplitude, or thelike. The receiver multiplies the received data signal by the inversecharacteristic of the channel response to compensate the received datasignal for any distortion.

Conventionally, an insertion method of the channel response calculationpreamble signal is fixed in a system. This method includes a method ofinserting a channel response calculation preamble signal at given timeintervals, a method of inserting at the head of a packet or frame senttoward a given user, and the like.

When a channel variation is large with respect to a packet length, thechannel response calculation result obtained from the channel responsecalculation preamble signal has a large error from the channeldistortion which is actually superposed on the data.

In order to reduce such error, when the frequency of insertion of thechannel response calculation preamble signal is increased, the datatransmission efficiency lowers when the channel variation is small.

As described above, in the conventional radio communication system andradio transmission apparatus, a large error is generated between thechannel response calculated from the channel response calculationpreamble signal and the channel distortion superposed on data due to achannel variation and, as a result, a reception error rate impairs. Inorder to reduce the error between the calculated channel response anddistortion superposed on data, if the frequency of output of the channelresponse calculation preamble signal is fixed to be high, the datatransmission efficiency lowers when the channel variation is small.

BRIEF SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a radiocommunication apparatus, system and method, which can accuratelycalculate a channel response and can reduce any transmission error evenwhen the channel variation is large.

According to one aspect of the present invention, a radio communicationmethod for transmitting a transmitted signal including a preamble signalused for calculating a channel response at a receiving side, comprisessetting an output frequency of the preamble signal based on a temporalchange of a radio propagation environment; and generating thetransmitted signal by inserting the preamble signal in accordance withthe output frequency.

With this method, when the channel variation is large, the frequency ofoutput of the channel response calculation preamble signal can beincreased, and distortion correction that traces the channel variationalong with an elapse of time can be made. On the other hand, when thechannel variation is small, the frequency of output of the channelresponse calculation preamble signal can be decreased, and the datatransmission efficiency can be improved.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 shows an example of the transmitted signal format of a radiocommunication system according to the present invention;

FIG. 2A is a graph showing a temporal variation of (signal) power as achannel response when the variation is large;

FIG. 2B is a graph showing a temporal variation of power as a channelresponse when the variation is small;

FIG. 3 shows an example of the transmitted signal format of a radiocommunication system according to the present invention;

FIG. 4 is a block diagram showing an embodiment of a radio transmissionapparatus according to the present invention;

FIG. 5 is a block diagram showing an embodiment of a radio transmissionapparatus according to the present invention;

FIG. 6 is a block diagram showing an embodiment of a radio transmissionapparatus according to the present invention; and

FIG. 7 is a block diagram showing another embodiment of a radiotransmission apparatus according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A radio transmission system and apparatus according to an embodiment ofthe present invention will be described hereinafter with reference tothe accompanying drawings.

FIG. 1 shows an example of the transmitted signal format of a radiocommunication system according to the present invention. A transmittedsignal is made up of a synchronization preamble signal, at least onechannel response calculation preamble signal (k signals in FIG. 1), anda plurality of data. The number of channel response calculation preamblesignals is variable in accordance with a channel variation. Thetransmitting side controls the frequency of the channel responsecalculation preamble signal in accordance with the channel variation.

FIGS. 2A and 2B are examples of graphs showing temporal variations ofpower as a channel response. The abscissa shows time, and the ordinateshows the intra-band average power of the channel response. FIG. 2Ashows a temporal variation of power when the temporal variation ofchannel response is large, and FIG. 2B shows a temporal variation ofpower when the temporal variation of channel response is small.

An appropriate channel format corresponding to the temporal variation ofpower of the channel response is shown below each graph that shows thetemporal variation of power of the channel response. As shown in FIG.2A, when the temporal variation of channel response is large, thefrequency of insertion of channel response calculation preamble signalis increased. A channel response to be calculated to compensate for adata distortion can be updated in correspondence with the temporalvariation of channel response. Therefore, a transmission error can bereduced. That is, when the temporal variation of channel response islarge, a data length m between two channel response calculation preamblesignals is set to be a small value, thus reducing transmission errors.

On the other hand, as shown in FIG. 2B, when the temporal variation ofchannel response is small, the frequency of insertion of the channelresponse calculation preamble signal is decreased, since the channelresponse need not be calculated frequently. As a result, the ratio ofthe total length of channel response calculation preamble signals to thetotal data length decreases. Hence, the data transmission efficiency canbe improved.

In brief, according to the present invention, the number of channelresponse calculation preamble signals inserted is varied in accordancewith the temporal variation of channel response.

One embodiment of the present invention will be described below. FIG. 3shows an example of the transmitted signal format in a radiocommunication system and radio transmission apparatus according to anembodiment of the present invention, and FIGS. 4, 5, and 6 are blockdiagrams.

In the transmitted signal format shown in FIG. 3, a control signal fieldwritten with control information, which is used to demodulate data, isadded to the transmitted signal format shown in FIG. 1. Since thiscontrol signal field is included, the receiving side can normallydemodulate data by recognizing control information written in thecontrol signal field.

In FIG. 3, the control signal field is inserted immediately after thefirst channel response calculation preamble signal, but its insertionposition is not particularly limited. The control signal field containsa channel response calculation preamble signal output frequency field.The transmitting side writes transmission frequency information ofchannel response calculation preamble signals in the channel responsecalculation preamble signal output frequency field. The receiving sidecan detect the transmission frequency of channel response calculationpreamble signals set at the transmitting side with reference to thecontents of the channel response calculation preamble signal outputfrequency field, and can calculate a channel response at a correcttiming.

That is, in the transmitted signal format shown in FIG. 3, since thesynchronization preamble signal and channel response calculationpreamble signal are always attached to the head of a packet. If thereceiving side can detect the output frequency, it can detect theposition of the next channel response calculation preamble signal. Theoutput frequency indicates that channel response calculation preamblesignals are inserted every n symbols, for example.

FIG. 4 shows an example of a radio transmission apparatus according tothe present invention. A base station 1 and terminal 2 have radiotransmission apparatuses with the same arrangement. A transmitter 11 ofthe base station 1 transmits a transmitted signal which containstransmitted data and channel response calculation preamble signals inthe transmission format shown in FIG. 3 from an antenna 12. Note that anoptimal number of channel response calculation preamble signals is setin accordance with channel variation information output from a channelvariation measurement unit 13.

The terminal 2 receives the transmitted signal transmitted from the basestation 1 by an antenna 22, and this signal is input to a receiver 24.The receiver 24 executes a reception process such as transmissiondistortion compensation and the like of a received signal using channelresponse calculation preamble signals, and outputs received data. Also,the receiver 24 outputs information used to measure a variation ofchannel response to a channel response variation measurement unit 23. Asan input signal to the channel response variation measurement unit 23, achannel response calculation result or the like of the received signalis used. The channel response variation measurement unit 23 measures avariation of channel response, and outputs the measurement result to atransmitter 21.

Note that the measurement method of the variation of channel responseincludes:

#1. a method of measuring a change in amplitude or phase from that ofthe previously calculated channel response every time a channel responseis calculated, in consideration of a subcarrier (one or a plurality ofsubcarriers) that includes the calculated channel response;

#2. a method of measuring the total (or average) power of respectivesymbols (as well as data), and measuring a change in amplitude (=power)or phase;

#3. a method of measuring a change in amplitude or phase of a pilotcarrier (for estimating a distortion) contained in each symbol; and thelike.

Furthermore, the transmitter 21 executes the same process as thetransmission process of the base station 1, and outputs a transmittedsignal to the base station 1 again. In this manner, a signal istransmitted between the base station 1 and terminal 2.

FIG. 5 is a block diagram for explaining the transmitter 11 of the radiotransmission apparatus shown in FIG. 4. Since the radio transmissionapparatuses of the base station 1 and terminal 2 have the samearrangement, the transmitter 11 of the base station 1 will be explainedas an example.

The signal which is received by the antenna 12 and has the transmissionformat shown in FIG. 3 is input to a receiver 14. The receiver 14executes a reception process of the received signal (to be describedlater), and outputs information used to measure a variation of channelresponse to a channel response variation measurement unit 13. As aninput signal to the channel response variation measurement unit 13, forexample, a channel response calculation result or the like is used. Thechannel response variation measurement unit 13 measures a variation ofchannel response, and outputs the measurement result to a channelresponse calculation preamble signal output frequency setting unit 31.

The channel response calculation preamble signal output frequencysetting unit 31 determines an optimal output frequency of channelresponse calculation preamble signals using the measurement result ofthe channel response variation, and informs a transmitted signalgenerator 32 and control signal field generator 33 of the outputfrequency of channel response calculation preamble signals.

The control signal field generator 33 writes information of the channelresponse calculation preamble signal output frequency in the controlsignal field.

Furthermore, transmitted data is converted into a data signal by a datasignal generator 34, and the data signal, a synchronization preamblesignal and channel response calculation preamble signals generated by apreamble signal generator 35, and the control signal field generated bythe control signal field generator 33 are input to the transmittedsignal generator 32. The transmitted signal generator 32 generates atransmitted signal based on the transmitted signal format shown in FIG.3, and outputs it from the antenna 12.

At this time, the channel response calculation preamble signals areinserted at the frequency set by the channel response calculationpreamble signal output frequency setting unit 31. In this manner, theoutput frequency of channel response calculation preamble signals can bechanged in accordance with a variation of channel response.

FIG. 6 is a block diagram for explaining the receiver 14 of the radiotransmission apparatus of the base station 1 shown in FIG. 4.

Referring to FIG. 6, the signal which is received by the antenna 12 andhas the transmission format shown in FIG. 3 is input to asynchronization unit 41. The synchronization unit 41 synchronizes thereceived signal, and outputs the synchronized received signal to achannel response calculation unit 42 and distortion compensation unit43. When a channel response is calculated using a channel responsecalculation preamble signal, the channel response calculation preamblesignal is input to the channel response calculation unit 42, and othersignals are input to the distortion compensation unit 43.

The channel response calculated by the channel response calculation unit42 is input to the distortion compensation unit 43. The distortioncompensation unit 43 compensates the received signal for any distortionusing the channel response calculated by the channel responsecalculation unit 42. Of the distortion-compensated signal, the controlsignal field is input to a control signal field analysis unit 44, whichreads out control information required to demodulate, and supplies it toa data demodulation unit 45. The data demodulation unit 45 demodulatesthe distortion-compensated signal, and outputs the demodulated signal asreceived data.

Furthermore, the control signal field analysis unit 44 reads outinformation indicating the channel response calculation preamble signaloutput frequency, and informs the distortion compensation unit 43 andchannel response calculation unit 42 of that channel responsecalculation preamble signal output frequency.

The channel response calculation unit 42 supplies a channel responsecalculation preamble signal to the channel variation measurement unit13, which measures the variation of channel response. The frequency ofchannel response calculation preamble signals to be inserted in thetransmitted signal format by the transmitter 11 is changed in accordancewith the measurement result.

In the radio transmission system with the above arrangement, since thefrequency of channel response calculation preamble signals to beinserted in the signal format to be transmitted is changed in accordancewith the channel variation, the channel response can be accuratelycalculated even when the channel variation is large, and a transmissionerror can be reduced.

Since the channel response calculation preamble signal output frequencyis sent to the distortion compensation unit 43 and channel responsecalculation unit 42, a channel response can be calculated at an accuratetiming, and accurate distortion compensation can be made.

Upon receiving this format, the channel response calculation unit 42 cancorrectly recognize the reception timing of a channel responsecalculation preamble signal based on the received channel responsecalculation preamble signal output frequency information, and cancalculate a channel response at a correct timing. Also, the distortioncorrection unit 43 can correctly recognize the distortion compensationtiming of data or the control signal field based on the received channelresponse calculation preamble signal output frequency. Thedistortion-compensated data is input to and demodulated by thedemodulation unit 45. In this way, since the receiving side accuratelyrecognizes the channel response calculation preamble signal outputfrequency, the data and control signal field can undergo distortioncorrection at correct timings. For this reason, even when thetransmitting side varies the channel response calculation preamblesignal output frequency, the data signal can be correctly demodulated.

The transmitted signal format shown in FIG. 3 contains the channelresponse calculation preamble signal output frequency field used toinform, from the transmitting side, the receiving side of the channelresponse calculation preamble signal transmission frequency. In place ofassuring the control signal field, as shown in FIG. 1, informationindicating the channel response calculation preamble signal outputfrequency may be contained in data.

In the above embodiment, the channel response calculation preamblesignal output frequency information is written in the transmittedsignal. However, in some cases, no channel response calculation preamblesignal output frequency information may be written in the transmittedsignal. In such case, the control signal field generator 33 of thetransmitter, and the control signal analysis unit 44 of the receiver maybe omitted.

When the channel response calculation preamble signal output frequencyis not available, a method of calculating a correlation between eachsymbol and a channel response calculation preamble signal on thereceiving side, and determining a symbol with high correlation as achannel response calculation preamble signal may be used.

As an input signal to the channel response variation measurement unit13, received signal strength (RSSI) information of a received signal maybe used. Using the RSSI, a variation of channel response is measured. Inthis case, as shown in FIG. 7, the channel response variationmeasurement unit 13 comprises a received signal strength measurementunit 13-1 and received signal strength variation measurement unit 13-2.The received signal strength measurement unit 13-1 measures the receivedsignal strength of a received signal from the antenna 12, and thereceived signal strength variation measurement unit 13-2 measures avariation of received signal strength. Furthermore, the result of thereceived signal strength variation measurement unit 13-2 is input to thechannel response calculation preamble signal output frequency settingunit 31 of the transmitter 11, thus setting the channel responsecalculation preamble signal output frequency in correspondence with thechannel variation.

In the embodiment shown in FIG. 5, the channel response calculationpreamble signal output frequency is set inside the transmitter of theradio transmission apparatus that generates a transmitted signal.However, some radio transmission system may receive channel responsecalculation preamble signal output frequency information from acommunication partner. In such case, the channel response variationmeasurement unit 13 and channel response calculation preamble frequencysetting unit 31 may be omitted. Channel response calculation preamblesignals may be inserted into the transmission format of a transmittedsignal based on the channel response calculation preamble signaltransmission frequency provided from the communication partner.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit and scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A radio communication apparatus comprising: a receiver receiving aradio signal; a first unit configured to obtain a channel response byuse of the radio signal; a second unit configured to measure amplitudeor phase from an obtained channel response by the second unit and todetermine a variation of a radio propagation environment based on ameasured result; a third unit configured to set a frequency of insertionof preamble signals based on the variation, the preamble signals beingused for obtaining a channel response at a receiving side; and atransmitted signal generator configured to generate a transmitted signalby inserting the preamble signals in accordance with the set frequencyof insertion and inserting an output frequency field indicating thefrequency of insertion of the preamble signals, the preamble signalsbeing separated with respect to time, wherein the third unit refers toan output frequency field contained in the radio signal received by thereceiver so as to control the frequency of insertion based on the outputfrequency field.
 2. The apparatus according to claim 1, wherein thefrequency of insertion of the preamble signals are increased as thevariation of the radio propagation environment gets larger.
 3. A radiocommunication method comprising: receiving a radio signal; obtaining achannel response by use of the radio signal; measuring amplitude orphase from an obtained channel response by the obtaining, anddetermining a variation of a radio propagation environment based on ameasured result; setting a frequency of insertion of preamble signalsbased on the variation, the preamble signals being used for obtaining achannel response at a receiving side; and generating a transmittedsignal by inserting the preamble signals in accordance with the setfrequency of insertion and inserting an output frequency fieldindicating the frequency of insertion of the preamble signals, thepreamble signals being separated with respect to time, wherein the thirdunit refers to an output frequency field contained in the radio signalreceived by the receiving so as to control the frequency of insertionbased on the output frequency field.
 4. The method according to claim 3,wherein the frequency of insertion of the preamble signals are increasedas the variation of the radio propagation environment gets larger.
 5. Aradio communication apparatus comprising: a receiver receiving a radiosignal having symbols; a first unit configured to calculate a totalpower of the symbols; a second unit configured to measure amplitude orphase of the total power, to determine a variation of a radiopropagation environment based on a measured result; a third unitconfigured to set a frequency of insertion of preamble signals based onthe variation, the preamble signals being used for obtaining a channelresponse at a receiving side; and a transmitted signal generatorconfigured to generate a transmitted signal by inserting the preamblesignals in accordance with the frequency of insertion, the preamblesignals being separated with respect to time, wherein the third unitrefers to an output frequency field contained in the radio signalreceived by the receiver so as to control the frequency of insertionbased on the output frequency field.
 6. The apparatus according to claim5, wherein the frequency of insertion of the preamble signals isincreased as the variation of the radio propagation environment getslarger.
 7. A radio communication apparatus comprising: a receiverreceiving a radio signal having symbols, the symbols each including apilot carrier; a first unit configured to measure amplitude or phase ofthe pilot carrier, to determine a variation of a radio propagationenvironment based on a measured result; a second unit configured to seta frequency of insertion of preamble signals based on the variation, thepreamble signals being used for obtaining a channel response at areceiving side; and a transmitted signal generator configured togenerate a transmitted signal by inserting the preamble signals inaccordance with the frequency of insertion, the preamble signals beingseparated with respect to time, wherein the second unit refers to anoutput frequency field contained in the radio signal received by thereceiver so as to control the frequency of insertion based on the outputfrequency field.
 8. The apparatus according to claim 7, wherein thefrequency of insertion of the preamble signals is increased as thevariation of the radio propagation environment gets larger.